Conventionally, the following method is well known for displaying a large screen image. Namely, the optical images corresponding to picture signals of respective colors are formed upon liquid crystal light valves of red, green and blue, and the optical images are irradiated with the red, green and blue lights segregated from a white light source such as a halogen lamp. Then the images are synthesized and are projected upon a screen by means of a projection lens. However, if a halogen lamp is used as a light source, a great amount of power is consumed and most of it is converted into heat. Thus there arises the problem of heat dissipation and hence the reduction of the size and the cost of the device is rendered difficult since an optical system for separating the white light into red, green and blue becomes necessary.
Thus, as a projection type display device for solving the above problems, a device shown in Japanese Laid-Open Patent (Kokai) No. 1-128689 is proposed. FIG. 47 is a structural diagram showing the conventional projection type display device disclosed in Japanese Laid-Open Patent (Kokai) No. 1-128689. In FIG. 47, the reference numeral 101 represents a red light source tube for emitting red light; the reference numeral 102 represents a green light source tube for emitting green light; and the reference numeral 103 represents a blue light source tube for emitting blue light. Each of the light source tubes of respective colors consists of a CRT type light source tube capable of emitting light of a single color. Further, the reference numeral 104 represents red light valve for modulating red light; the reference numeral 105 represents green light valve for modulating green light; and the reference numeral 106 represents blue light valve for modulating blue light. The reference numeral 107 represents a synthesis prism for synthesizing the modulated red, green and blue light beams. The projection lens 108 projects the synthesized light. The light-emitting portions of the light source tube of the respective colors have dimensions sufficient to cover the image forming portions of the liquid crystal light valves 104, 105, and 106.
Further, FIG. 48 is an enlarged sectional view of a CRT type light source tube constituting the light source tubes for the respective colors. The CRT type light source tube of FIG. 48 has a structure similar to that of a cathode ray tube (CRT), and includes a heater 50, a cathode 49, a first grid 44, a second grid 43, a third grid 42, and an over-convergence lens 48 accommodated within a glass bulb 46. On the front interior surface of the glass bulb 46 is formed a fluorescent surface 41. Further, upon the interior surface of the fluorescent surface 41 is formed an aluminum back layer 51 which serves as a high voltage electrode for acceleration. Furthermore, the glass bulb 46 is attached to the base portion 45, and the various electrodes are connected to the exterior via the base portion 45.
Next, the operation is described.
In the case of the CRT type light source tube of FIG. 48, the heater 50 is heated and appropriate voltages are applied on the first grid 44, the second grid 43, and the third grid 42, thereby forming an over-convergence lens 48 by means of the respective grids. A high voltage of about 10 KV is applied to the aluminum back layer 51. As a result, the electron beam 47 emitted from the cathode 49 is over-converged and then is accelerated and projected upon the fluorescent surface, which then emits visible light.
Thus, the red light is emitted from the red light source tube 101 and an image of the red component is displayed on the red light valve 104. The image of the red light transmitted through the red light valve 104 is reflected by the synthesis prism 107. The green light is emitted from the green light source tube 102 and an image of the green component is displayed on the green light valve 105. The image of the green light transmitted through the green light valve 105 is transmitted through the synthesis prism 107. The blue light is emitted from the blue light source tube 103 and an image of the blue component is displayed on the blue light valve 106. The image of the blue light transmitted through the blue light valve 106 is reflected by the synthesis prism 107. The image thus synthesized by the synthesis prism 107 is magnified by the projection lens 108 and the projection light 109 is projected on a screen (not shown).
Since the conventional projection type display device has a structure as described above, there exists a gap between the respective light valves and the light source tubes. Thus most of the light emitted from the light source, which is completely diffusing, fails to arrive on the light valves and the projection lens, and thus is not guided to the screen. As a result, the utilization efficiency of the emitted light is low.